This book isn’t up to the (extremely high) standards set by the official material for 13th Age, but there’s enough good stuff to make it worthwhile. Four stars would be a shade too generous.
For players, the book offers two full character classes, two races, three pages of magic items and a dozen feats, plus four clerical domains hidden away in the bestiary. The clear standout is the realm-walker, a five-star jewel of a new class: high magic without spells, and a nicely-gauged balance of direct combat and support options. It won’t fit into all games, but if you’re interested in including the planes as a campaign element, it should fit into yours.
All the rest of the book – 80% or more – is for GMs. Perhaps the biggest disappointment is how very little it contains about the overworld: literally no more than a page or so detailing one single location, plus one PC feat and a few NPCs. I’d expected suggestions about different ways to present the overworld, and probably planar structure as well, pointing in incompatible directions along the usual 13A pattern. Nothing of the sort.
On the one hand, it’s a reviewing sin to complain that the author didn’t write what you think they should have. On the other hand, “overworld” is right there in the title.
Similarly, there aren’t general guidelines on adventuring in other planes. What it does have is worked examples – pretty good ones – integrated into the adventures. These are about 30% of the book: one has little planar material, the other two cross multiple planes and are surprisingly dependent on random incident. This is also where the other planes strut their stuff, so maybe the touristy elements aren’t so surprising.
Another 30% is two sandbox settings. The Carrow Hills is adventurer-tier, and relatively scanty in otherplanar suggestions. The City at the Edge of Dawn is a pocket-dimensional answer to Sigil, champion-tier on average although you could push it up or down. Both are described in enough detail to get the GM started, with a nice selection of NPC ideas, and a considerable array of different adventure seeds
The rest of the GM material is a substantial bestiary, written as shorter core rules entries rather than the more developed format of the Bestiaries. Finally, there’s miscellaneous grab-bag similar to the random stuff at the end of 13 True Ways. Some of the bits, here and elsewhere, are in the gonzo F20 tradition going back to the early days of the hobby, but if you don’t care for that material it’s easy to excise.
Of the official 13A books, this one is most closely comparable to The Book of Demons, even to a roughly comparable player/GM balance. The planes are, to my taste, a more interesting subject for a book, but the narrower focus is handled better there than the potentially much broader canvas here. The adventures are OK but don’t really measure up to the same standard. The bestiary is larger, but probably less well designed and certainly less evocative. The sandbox material in The Overworld and Beyond does beat out the Floating Market and the citadels: it’s more broadly useful and ends up with more plot hooks and inspirational elements.